This invention relates to processes for coating sewing thread, and in particular to processes for coating sewing thread using solventless systems.
Sewing thread is typically constructed from multiple continuous filament multifilament plies which are individually twisted in one direction and then combined by twisting in the opposite direction to produce a multiple ply final thread. In general, this causes the separate plies to act as a single unitary ply during the sewing process.
In many highly demanding industrial environments, it can be necessary to further treat sewing thread prior to its use. Such treatments can improve the integrity and retention of individual filaments within the sewing thread under conditions of high abrasion, improve the adhesion of the plies or individual filaments to each other in monocord or multicord constructions, and improve durability of the thread in its final end use. In these instances, the thread is coated with a bonding agent in the form of a lacquer or other plastic material which essentially forms a solid yet flexible film or sheath surrounding the thread. This allows the thread to retain substantial flexibility because the individual filaments of the thread retain the ability to have some movement relative to each other.
If the bonding agent fully penetrates the cross-section of the thread, however, the individual filaments are bound to each other and a stiff thread results. Such a stiff thread performs similar to a monofilament thread and can be unacceptable for many end use applications.
Conventionally, sewing thread is coated by passing the thread through a suitable resin in a solvent and then through a heating oven which evaporates the solvent and leaves the film. This operation, however, can be slow and releases organic solvent materials into the atmosphere. In addition, the film or sheath tends to flake off the sewing thread when used in demanding applications. The flaking is highly undesirable because it produces a visible dandruff-like deposit on the product. Further, energy required to remove the solvent can increase production costs.
Solventless systems can avoid many problems associated with solvent based coatings. In an exemplary solventless system, sewing thread is coated with a prepolymer material (such as a monomer plus a catalyst) which is capable of reacting to form a film when exposed to ultraviolet (UV) radiation. However, radiation curable systems also tend to flake off the sewing thread when the sewing thread is used in demanding applications. Other problems which can be associated with the use of many solventless systems include incomplete cure, tackiness, low adhesion and low production speeds.
The present invention provides processes for coating sewing thread using solventless systems. The solventless systems are generally more environmentally acceptable than conventional solvent based systems. In addition, the solventless systems can be applied to the thread and then cured in an in-line process at a greatly increased rate as compared to solvent based processes, which can reduce production costs. Solvent or water does not have to be removed from the solventless system after coating, thus reducing energy costs. In addition, sewing thread can be coated using smaller sized equipment, thus reduction production space.
In contrast to prior solventless systems, however, the coated sewing thread exhibits improved resistance to flaking and powdering. In addition, the coated sewing threads of the invention can have excellent adhesion properties which can protect the thread surface during demanding high speed industrial applications.
In the invention, a radiation curable material or resin system is applied in-line to a continuous threadline. When the thus coated thread is exposed to radiation, the radiation initiates polymerization or cure of the resin. In contrast to prior solventless systems, however, the radiation initiates a reaction that is self sustaining following initiation, as explained below.
Preferably, the radiation curable system is a cationic initiated system in which the radiation initiates a self sustaining crosslinking reaction following initiation, in contrast to many radiation cured polymers which are based upon a free radical mechanism. In the latter type mechanism, it is currently believed that the reaction only proceeds in the presence of UV radiation. However, in the case of a sewing thread, it is believed that at least a portion of the resin applied to the thread is shielded from the UV radiation by the individual filaments in the thread. Thus, for free radical initiated UV resins, the shielded portion of the resin is never fully reacted and hardened.
In contrast, for cationic initiated resins, the shielded portion of the resin is hardened as a result of the self sustaining thermal reaction initiated by the UV radiation even though the shielded portion of the resin is never irradiated directly. As a result, the cationic initiated systems cure or react more completely and as a result do not suffer from the tacking and flaking problems associated with free radical radiation curable resins.
In the invention, to coat the sewing thread, the sewing thread is passed through a cavity in a coating apparatus which contains the radiation curable material under pressure. The radiation curable material is applied to the thread in the cavity using a xe2x80x9ccontact coatingxe2x80x9d process in which the pressurized radiation curable resin is applied to the exterior of the sewing thread as the sewing thread is contacted by a surface so as to impregnate resin into the periphery of the sewing thread. In one embodiment of the invention, contact coating is achieved by employing a coating die having an orifice of smaller diameter than the diameter of the sewing thread. Alternatively, a deformable porous media can be provided in the die cavity so that it surrounds and contacts the sewing thread as it passes through the cavity. As a result of contact coating, the resultant sewing thread exhibits a thin layer of radiation curable material that has been impregnated into the periphery of the sewing thread.
Following curing of the radiation curable coating, the sewing thread of the invention differs structurally from conventional organic solvent based coated sewing thread because the radiation curable composition is applied so that the composition penetrates into the periphery of the sewing thread, preferably to a depth of one to about three single filament layers (or diameters), and the peripheral filaments are bonded to each other and in some cases to the next interior level of filaments. Although the composition penetrates the thread, the degree of penetration is controlled to prevent the thread from becoming unduly stiff. Thus, a continuous sheath of resin is not formed around the sewing thread, in contrast with conventional coated sewing thread; instead, the coating extends into the periphery of the thread. This can advantageously minimize or prevent stripping of the coating caused by abrasive forces such as are encountered in sewing processes.